Are you photos showing up on this other site or are they just linking to your site? You can stop them from linking to your site but you can stop the traffice from their site. You would stop the traffice from their site by using htaccess.
Where xx.xx.xx.xx is the IP address of the site in question that you want to block. Keep in mind this "could" block a legitimate request as someone could share the same ip address. If your photos are showing up on their site meaning they are hotlinking your photos then do a google search for how to prevent hotlinking with htaccess. You will get a lot of results.
deny from xx.xx.xx.xx
allow from all